1. Field of the Invention
The present invention relates to a tire condition monitoring device that transmits information for monitoring a condition of a tire from a transmitter installed in the tire to a receiver installed in a vehicle body.
2. Description of the Related Art
A tire pressure monitoring system (TPMS) that detects a pressure of a tire using a sensor, transmits the pressure to a device on the vehicle side, and monitors whether the air pressure is abnormal is known.
In general, in a device on the tire side, a transmitter is controlled so that a transmission interval of a radio signal becomes long at the time of stop of a vehicle in order to reduce consumption of a battery. Further, in the device on the vehicle side, a receiver is controlled to operate intermittently in order to save power at the time of stop of an engine. Therefore, both the transmitter and the receiver may operate intermittently, and it becomes necessary to match timings of transmission and reception.
In a tire pressure monitoring device described in Japanese Unexamined Patent Application Publication No. 2005-153641 below, a data transmission time of a transmitter is longer than an intermittent startup time of a receiver. Accordingly, in a case in which data transmission from the transmitter is performed, the receiver can reliably receive data.
However, if an intensity of radio waves or the like is regulated under regulations, the transmitter cannot take a long time of one transmission. Further, if the transmission time is long, the power of the transmitter increases. Accordingly, a transmission output should be decreased so that the power does not exceed a rated power. As a result, a sufficient reception sensitivity may not be obtained in a receiver on the vehicle side. In such a case, a method of increasing the transmission time as described in Japanese Unexamined Patent Application Publication No. 2005-153641 cannot be adopted.
Instead of increasing the transmission time, decreasing an interval of a reception operation in the receiver may be considered. However, for example, in a case in which the receiver is also used as another system (for example, a keyless entry system), the method in Japanese Unexamined Patent Application Publication No. 2005-153641 cannot be adopted since the interval of the reception operation of the receiver cannot be freely changed.
A method of providing a high-precision real-time clock in both of the transmitter and the receiver may be considered in order to match a transmission timing of the transmitter with a reception timing of the receiver. However, this method has a problem that a circuit configuration is complicated and costs increase.